1. Field of the Invention
The present invention relates to an apparatus for injecting fuel into an engine, and more particularly, to a computer controlled fuel injection apparatus which optimizes fuel injection control based on variations in detected fuel pressure.
2. Description of the Related Art
Two of the main components in conventional fuel injection systems used in diesel and other types of engines are a fuel injection pump 151 and injection nozzles 152 as shown in FIG. 30. Each injection nozzle 152 operates to inject fuel, supplied under high pressure by the injection pump 151, into a combustion chamber 154 of an engine 153. The amount of fuel injected into the combustion chamber 154 is variously controlled by fuel injection timing according to changing engine operating parameters. The fuel injection apparatus used in the electronically controlled diesel engine, for example, utilizes a fuel pump 151 activated by a plunger 155 to compress fuel in a high pressure chamber 156 inside the pump 151. The high-pressure fuel is then fed to the injection nozzle 152 from which it is injected into the combustion chamber 154.
A computer 157 or other type of electronic device typically controls the injection pump 151. The computer 157 or electronic device first computes a desired or target value of fuel to be injected in the engine 153 based on detected engine operating conditions. Then, based on the computed target value, the computer 157 controls an electromagnetic coil 158 provided in the injection pump 151 to open a spill valve 159 (or a spill ring in other pump types). Next, the high pressure chamber 156 opens to a fuel chamber 160 allowing fuel to spill into the fuel chamber 160 from the high pressure chamber 156. The amount of fuel supplied during any single injection operation depends directly on the time which the injector pump 151 begins and ends its supply of pressurized fuel to the injection nozzle 152 and combustion chamber 154.
The computer 157 accomplishes this by computing a target value for injection timing in accordance with various engine operating conditions. The computed target value is then used by the computer 157 to control a timer device 161 provided in the injection pump 151, which in turn, controls the movement of the plunger 155. In this way, the computer 157 controls the action of the plunger 155 to begin the supply of pressurized fuel from the injection pump 151 to the injection nozzle 152, and ultimately, to the combustion chamber 154.
Even with such electronically precise control over the timing and amount of fuel injection, time dependent mechanical changes as well as changes in fuel characteristics tend to compromise the performance of fuel injection control. Instances of this occur given variations in fuel viscosity and mechanical component part degradation. In particular, when the temperature of fuel fluctuates, fuel viscosity undergoes a significant change. This change directly affects fuel injection. In addition, over long periods of time, the component parts of the fuel injector will normally degrade or show signs of wear. Unless these factors are considered in the timing control of the fuel injection, the actual amount of fuel injected and its timing will inevitably vary from desired target values. These factors, in turn, compromise engine emissions, e.g., increasing the amounts of nitrogen oxides (NOx) and other harmful compounds exhausted from the engine 153.
Japanese Unexamined Patent Publication No. 62-291453 proposes a simple structure to control the amount of fuel injected by an injection pump, that accounts for variations in fuel viscosity. In this apparatus, a hydraulic timer has a hydraulic piston and a cylinder provided in the injection pump. The computer first determines the degree of fuel viscosity based on detected changes in the speed of the reciprocating piston, and then, utilizes the detected fuel viscosity to ultimately adjust the fuel injection control.
According to the control apparatus in the '453 unexamined patent publication, however, fuel viscosity determinations are made irrespective of considerations such as variations in the particular manufacturing tolerance of the piston and cylinder in any given timing apparatus. Likewise, nothing is provided in this fuel injection control system which accounts for the wearing out or the mechanical failure of timing apparatus piston and cylinder. Such failures are often directly dependent on engine speed and load. Consequently, the control apparatus disclosed in the '453 unexamined patent publication is ill-suited to make precise fuel viscosity determinations under various engine speeds and loads. This, in effect, prevents precise fuel injection control.